Traditionally, Catholics think of mission efforts as sending religious women and men to pagan territories. The pope emeritus would suggest a wider view:
375. His Holiness Benedict XVI has confirmed that mission ad gentes is opening to new dimensions.
Thus, the area of the “missio ad gentes” appears to have been considerably extended and cannot be defined solely on the basis of geographical or juridical considerations; indeed, the missionary activity of the People of God is not only intended for non-Christian peoples and distant lands, but above all for social and cultural contexts and hearts.(Benedict XVI, Address on Fortieth Anniversary of Ad Gentes, March 11, 2006)
He might be thinking of post-Christian Europe, where faith lags and many baptized Christians have minimal commitment to elements of faith, traditional or evangelical. Other might see themselves as members, rather than active elements of the Body. Cultures long considered Christian have new elements, some harmonious with faith and others antithetical to it.
From Vatican II:
376. At the same time, the world expects of our Latin American and Caribbean church a more significant commitment to the universal mission on all continents. In order to avoid falling into the trap of becoming closed in on ourselves, we must be formed as missionary disciples without borders, willing to go “to the other shore,” there where Christ is still not recognized as God and Lord, and the church is not yet present.(Cf. Ad Gentes 6)
Remember, you can reference an English translation of the 2007 document from the Aparecida Conference here.